Dental caries incidence is higher in adults than children. Recent recommendations from the Centers for Disease Control and Prevention and an NIH Consensus Development Conference on dental caries call for trials of caries prevention methods in caries-active adults. Little is known about the effectiveness of caries prevention methods in adults, especially for adults who are at elevated risk for developing caries lesions. This proposal describes a clinical trial of an intervention to prevent dental caries in caries-active adults that is responsive to the CDC and Consensus Conference recommendations. The Xylitol for Adult Caries Trial (X-ACT) is a three-year, multi-center, randomized, placebo-controlled, double-blind study to assess the effectiveness of xylitol lozenges in reducing the incidence of coronal and root caries in caries-active adults. Participants will be 750 adults (ages 25-75) with current or recent caries lesions who are dental school patients at one of the three clinical centers in Chapel Hill, North Carolina;Birmingham, Alabama;and San Antonio, Texas. This intervention is being tested because it represents a potentially effective caries prevention method with a high likelihood of adoption by dental practitioners as an "add on" caries preventive method for their cariesactive patients. The potential for adoption of the intervention is an important consideration, because current data suggest that caries-active adults are underexposed to caries-preventive treatment in dental practices. This Clinical Center application from the University of Texas Health Science Center (UTHSC) San Antonio is part of the coordinated set of applications comprising the X-ACT study. Accompanying this application are applications from the Study Chair, Data Coordinating Center, and two other clinical centers. A Manual of Procedures appears as an appendix to this Study Chair application. Relevance to public health: Tooth decay is a serious problem for many adults. Little research has been done on methods to prevent tooth decay in adults. This study would test how well a lozenge to be taken several times a day works to prevent tooth decay in adults who are prone to tooth decay.